The present invention relates to a percussive down-the-hole hammer for rock drilling, and a drill bit used therein.
A prior art drill bit for a down-the-hole hammer is disclosed in U.S. Pat. No. 6,062,322. The drill bit comprises an extended anvil portion on which a piston impacts repeatedly to advance the down-the-hole hammer through the rock. However, when constructing a large diameter hammer having a diameter of at least 10 inches, the drill bit becomes relatively large and expensive. It would be desirable to shorten the drill bit and thus provide a more compact hammer, which is relatively simple to manufacture, while still providing for a high efficiency.
One object of the present invention is to provide an efficient down-the-hole hammer which is compact, relatively easy to manufacture, and which contains a minimum of parts.
An additional object is to provide a drill bit for a down-the-hole hammer, which is economical to produce.
A first aspect of the present invention relates to a down-the-hole percussive hammer for rock drilling. The hammer comprises a generally cylindrical casing, and a drill bit disposed at a front end of the casing. The drill bit comprises a front portion which protrudes from the casing and includes a forwardly facing cutting surface, and a center longitudinal passage extending forwardly through a rearwardly facing rearwardmost end surface of the drill bit. The passage communicates with the front surface and includes a rearwardly facing impact surface spaced forwardly from the rearwardmost end surface. The hammer further includes a top sub mounted in an upper portion of the casing, and a hollow feed tube mounted to the top sub and extending downwardly along a longitudinal center axis of the casing. The feed tube defines a center passage adapted to conduct pressurized air. The hammer also includes a piston mounted in the casing longitudinally behind the drill bit for reciprocation in a longitudinal direction. The piston includes an axial throughhole slidably receiving the feed tube, and a front portion sized to enter the center passage of the drill bit. The front portion of the piston includes a front end defining a forwardly facing striking surface for striking the impact surface during each forward stroke of the piston.
Preferably, the impact surface is spaced from the rearwardmost end surface of the drill bit by a distance of at least ten percent of a total longitudinal length of the drill bit.
The invention also pertains to the drill bit per se.